1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Modern wireless RF transmitters for applications, such as cellular, personal, and satellite communications, employ digital modulation schemes such as frequency shift keying (FSK) and phase shift keying (PSK), and variants thereof, often in combination with code division multiple access (CDMA) communication. Independent of the particular communications scheme employed, the RF transmitter output signal, sRF(t), can be represented mathematically assRF(t)=r(t)cos(2πfct+θ(t))  (1)where fc denotes the RF carrier frequency, and the signal components r(t) and θ(t) are referred to as the envelope and phase of sRF(t), respectively.
Some of the above mentioned communication schemes have constant envelope, i.e.,r(t)=R, and these are thus referred to as constant-envelope communications schemes. In these communications schemes, θ(t) constitutes all of the information bearing part of the transmitted signal. Other communications schemes have envelopes (amplitudes) that vary with time and these are thus referred to as variable-envelope communications schemes. In these communications schemes, both r(t) and θ(t) constitute information bearing parts of the transmitted signal.
The most widespread standard in cellular wireless communications is currently the Global System for Mobile Communications (GSM). The GSM standard employs Gaussian Minimum Shift Keying (GMSK), which is a constant-envelope binary modulation scheme allowing raw transmission at a maximum rate of 270.83 kilobits per second (kbps). Even higher data rates are achieved in the specification of the Enhanced Data rates for GSM Evolution (EDGE) cellular telephony standard by selectively applying a 3π/8 offset, 8-level PSK (8-PSK) modulation scheme. With this variable-envelope communication scheme, the maximum bit rate is tripled compared to GSM, while the chosen pulse shaping ensures that the RF carrier bandwidth is the same as that of GSM, allowing for the reuse of the GSM signal bandwidths.
As mentioned above, the 8-PSK modulation scheme of EDGE is an example of a variable envelope communications scheme. A common transmitter used in such variable-envelope modulation communications schemes is the polar transmitter. In a typical polar transmitter architecture, digital baseband data enters a digital processor that performs the necessary pulse shaping and modulation to some intermediate frequency (IF) carrier fIF to generate digital amplitude-modulated and digital phase-modulated signals. The digital amplitude-modulated signal is input to a digital-to-analog converter (DAC), followed by a low pass filter (LPF), along an amplitude path, and the digital phase-modulated signal is input to another DAC, followed by another LPF, along a phase path. The output of the LPF on the amplitude path is an analog amplitude signal, while the output of the LPF on the phase path is an analog phase signal. The analog phase signal is input to a phase-locked loop (PLL) to enable the phase of the RF output signal to track the phase of the analog phase signal. The RF output signal is modulated in a non-linear power amplifier (PA) by the analog amplitude signal. Thus, in polar transmitter architectures, the phase component of the RF signal is amplified through the non-linear PA while the amplitude modulation is performed at the output of the PA.
In practice, the power spectrum emitted from an EDGE polar transmitter will not be ideal due to various imperfections in the RF transmitter circuitry. Thus, quality measures of the transmitter performance have been established as part of the EDGE standard and minimum requirements have been set. One quality measure that relates to the RF signal power spectrum is the so-called spectral mask. This mask represents the maximum allowable levels of the power spectrum as a function of frequency offset from the RF carrier in order for a given transmitter to qualify for EDGE certification. In other words, the spectral mask requirements limit the amount of transmitter signal leakage into other users' signal spectrum. For example, at a frequency offset of 400 kHz (0.4 MHz), the maximum allowable emission level is −54 dB relative to the carrier (dBc). Another RF transmitter quality measure of the EDGE standard is the modulation accuracy, which relates the RF transmitter modulation performance to an ideal reference signal. Modulation accuracy is related to the so-called error vector magnitude (EVM), which is the magnitude of the difference between the actual transmitter output and the ideal reference signal. The error vector is, in general, a complex quantity and hence can be viewed as a vector in the complex plane. Modulation accuracy is stated in root-mean-square (RMS), 95th percentile, and peak values of the EVM and is specified as a percentage. For a given transmitter to qualify for EDGE certification, the RMS EVM must be less than 9%, the 95th percentile of EVM values must be less than 15%, and the peak EVM value must be less than 30%.
One component of the RF circuitry that significantly affects the performance of the transmitter is the power amplifier. There are three main sources of nonlinearities in most power amplifiers that contribute to the degradation of both the spectral mask and the EVM. The first source is known as LO feed-through (LOFT). Within a polar transmitter, the RF phase-modulated signal is typically generated by up-converting the IF phase-modulated signal to the desired RF signal using a local oscillator generator (LO). As such, the RF phase-modulated signal is commonly referred to as the LO signal. Ideally, the output of the power amplifier includes only the product of the LO signal and the amplitude-modulated signal. However, due to imperfections in the power amplifier, a portion of the LO signal may also appear at the power amplifier output. This leakage of the LO signal affects the performance of the transmitter by increasing both the spectral mask and the EVM.
The other sources of nonlinearities in the power amplifier are AM-AM distortion and AM-PM distortion. As the amplitude of the output signal varies, distortion is added to both the amplitude-modulated (AM) signal and the phase-modulated (PM) signal. For example, since the amount of LO leakage changes with the amplitude level of the output signal, when amplitude modulation is applied to the power amplifier, there is a variation in the carrier's phase due to the leakage that is a function of the carrier's envelope (amplitude). This effect is known as AM-PM distortion, and is critical when the power amplifier operates at high output power level.
Therefore, what is needed is a polar transmitter architecture capable of compensating for nonlinearities in the power amplifier.